Emmi Ruokokoski

Observation of Dirac monopoles in a synthetic magnetic field

Magnetic monopoles—particles that behave as isolated north or south magnetic poles—have been the subject of speculation since the first detailed observations of magnetism several hundred years ago. Numerous theoretical investigations and hitherto unsuccessful experimental searches have followed Dirac’s 1931 development of a theory of monopoles consistent with both quantum mechanics and the gauge invariance of the electromagnetic field. The existence of even a single Dirac magnetic monopole would have far-reaching physical consequences, most famously explaining the quantization of electric charge. Although analogues of magnetic monopoles have been found in exotic spin ices and other systems, there has been no direct experimental observation of Dirac monopoles within a medium described by a quantum field, such as superfluid helium-3 (refs 10, 11, 12, 13). Here we demonstrate the controlled creation of Dirac monopoles in the synthetic magnetic field produced by a spinor Bose–Einstein condensate. Monopoles are identified, in both experiments and matching numerical simulations, at the termini of vortex lines within the condensate. By directly imaging such a vortex line, the presence of a monopole may be discerned from the experimental data alone. These real-space images provide conclusive and long-awaited experimental evidence of the existence of Dirac monopoles. Our result provides an unprecedented opportunity to observe and manipulate these quantum mechanical entities in a controlled environment.

Observation of isolated monopoles in a quantum field

Making a monopole in an atomic gas Some “grand unified theories” of physics predict the existence of the so-called magnetic monopole. No such particles have been seen, but analogs of monopoles may be observable in quantum fluids. Ray et al. created such an analog in a gas of ultracold 87Rb atoms with three spin states at their disposal. The authors used a protocol involving external magnetic fields with particular spatial distributions to create and observe a monopole-like spin texture in the gas. Science, this issue p. 544 An analog of the magnetic monopole has been created and observed in an ultracold gas of rubidium-87 atoms. Topological defects play important roles throughout nature, appearing in contexts as diverse as cosmology, particle physics, superfluidity, liquid crystals, and metallurgy. Point defects can arise naturally as magnetic monopoles resulting from symmetry breaking in grand unified theories. We devised an experiment to create and detect quantum mechanical analogs of such monopoles in a spin-1 Bose-Einstein condensate. The defects, which were stable on the time scale of our experiments, were identified from spin-resolved images of the condensate density profile that exhibit a characteristic dependence on the choice of quantization axis. Our observations lay the foundation for experimental studies of the dynamics and stability of topological point defects in quantum systems.

Stationary states of trapped spin-orbit-coupled Bose-Einstein condensates

We numerically investigate low-energy stationary states of pseudospin-1 Bose-Einstein condensates in the presence of Rashba-Dresselhaus-type spin-orbit coupling. We show that for experimentally feasible parameters and strong spin-orbit coupling, the ground state is a square vortex lattice irrespective of the nature of the spin-dependent interactions. For weak spin-orbit coupling, the lowest-energy state may host a single vortex. Furthermore, we analytically derive constraints that explain why certain stationary states do not emerge as ground states. Importantly, we show that the distinct stationary states can be observed experimentally by standard time-of-flight spin-independent absorption imaging.

Ground-state Dirac monopole

We show theoretically that a monopole defect, analogous to the Dirac magnetic monopole, may exist as the ground state of a dilute spin-1 Bose-Einstein condensate. The ground-state monopole is not attached to a single semi-infinite Dirac string but forms a point where the circulation of a single vortex line is reversed. Furthermore, the three-dimensional dynamics of this monopole defect is studied after the magnetic field pinning the monopole is removed and the emergence of antimonopoles is observed. Our scheme is realizable with the current experimental facilities.

Decay of an isolated monopole into a Dirac monopole configuration

We study numerically the detailed structure and decay dynamics of isolated monopoles in conditions similar to those of their recent experimental discovery. We find that the core of a monopole in the polar phase of a spin-1 Bose–Einstein condensate contains a small half-quantum vortex ring. Well after the creation of the the monopole, we observe a dynamical quantum phase transition that destroys the polar phase. Strikingly, the emergent ferromagnetic order parameter exhibits a Dirac monopole in its synthetic magnetic field.

Monopoles in Spinor Bose-Einstein Condensates

We describe the creation and observation of Dirac monopoles in a synthetic magnetic field, and also provide evidence of isolated topological point defects in the order parameter of a spinor Bose-Einstein condensate.